In a code division multiple access (CDMA) communication system, a CDMA forward link signal which is a combination of several data channels is modulated onto a carrier signal to form a CDMA carrier signal. Multiple CDMA carrier signals are combined together in a combiner to form a combined signal before being transmitted from an antenna. The CDMA carrier signals each have a different center frequency, and it may be as close as a channel bandwidth of the system. Generally, each carrier signal is passed through a band-pass filter. These filters should have a very sharp roll-off, which is a characteristic of a high quality factor, to filter coupled components of other carriers at frequencies away from the center frequency, and provide very low attenuation loss at the center frequency. Generally, the filters should have a high loaded quality factor and low attenuation loss at the center frequency. In a CDMA system where channel bandwidth is in the order of 1.2 MHz, cavity filters may be used. Cavity filters typically provide loaded filter quality factor of at least 50,000. In the prior art, cavity filters normally meet the requirements of high filter quality factor and low center frequency insertion loss, however, they are very bulky and expensive.
As the physical size and cost of the CDMA system hardware is reduced, the use of bulky cavity filters becomes highly undesirable. Moreover, cavity filters are expensive to manufacture, and very often, do not provide consistent performance among the manufactured filters. As a result, ceramic filters that provide the option of small size and consistent performance are used as a substitute for cavity filters. However, ceramic filters do not provide high filter loaded quality factor and low attenuation loss. To maintain low ceramic filter attenuation loss, the carrier signals being combined are forced to be at frequencies far away from each other. This would allow a degradation in the ceramic filter roll-off attenuation which is a degradation in quality factor while maintaining low attenuation loss. For example, carrier signals at alternate channel frequencies are generally combined with the use of cavity filters. In contrast, with the use of ceramic filters, the carrier signals that are combined have a frequency separation of about several frequency bandwidths of the system. The inability to combine carrier signals at close frequency range with the use of ceramic filters would greatly reduce the capacity of a CDMA system.
Therefore, there is a need for an improved method and apparatus of combining carrier signals.